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-rw-r--r--src/Conftrack.hs172
-rw-r--r--src/Conftrack/Source.hs16
-rw-r--r--src/Conftrack/Source/Aeson.hs7
-rw-r--r--src/Conftrack/Source/Trivial.hs1
-rw-r--r--src/Conftrack/Source/Yaml.hs1
-rw-r--r--src/Conftrack/Value.hs18
6 files changed, 205 insertions, 10 deletions
diff --git a/src/Conftrack.hs b/src/Conftrack.hs
index e1652b1..4e1f17a 100644
--- a/src/Conftrack.hs
+++ b/src/Conftrack.hs
@@ -9,19 +9,39 @@
{-# LANGUAGE RecordWildCards #-}
{-# OPTIONS_GHC -Wno-incomplete-uni-patterns #-}
+{-|
+Module: Conftrack
+Stability: experimental
+
+A typeclass-based library for reading in configuration values from multiple sources,
+attempting to be simple, avoid unecessarily complex types, and be able to track where
+each value came from.
+
+-}
module Conftrack
- ( Config(..)
- , Warning(..)
- , runFetchConfig
+ ( -- * How to use this library
+ -- $use
+
+ -- * Defining a configuration format
+ Config(..)
, readValue
, readOptionalValue
, readRequiredValue
, readNested
, readNestedOptional
+ -- * Defining sources
, SomeSource
- , ConfigError(..)
- , Key(..)
+ -- * Reading a config
+ , runFetchConfig
+ , Fetch
+ -- * Parsing config values
, Value(..)
+ , ConfigValue(..)
+ -- * Basic types
+ , Key(..)
+ , Warning(..)
+ , ConfigError(..)
+ -- * Utilities
, configKeysOf
, key
) where
@@ -43,6 +63,7 @@ import Data.Map (Map)
import qualified Data.Map.Strict as M
+-- | A class to model configurations. See "Conftrack"'s documention for a usage example
class Config a where
readConfig :: Fetch a
@@ -54,6 +75,16 @@ data FetcherState = FetcherState
, fetcherErrors :: [ConfigError]
}
+-- | A value of type @Fetch a@ can be used to read in a value @a@, with configuration
+-- sources handled implicitly.
+--
+-- Note that this is an instance of 'Applicative' but not 'Monad'. In practical terms
+-- this means that values read from the configuration sources cannot be inspected while
+-- reading the rest of the config, and in particular which keys are read cannot depend
+-- on another key's value. This allows for introspection functions like 'configKeysOf'.
+--
+-- For configuration keys whose presence depends on each other, use
+-- 'Conftrack.readNestedOptional' to model similar behaviour.
newtype Fetch a = Fetch (FetcherState -> IO (a, FetcherState))
deriving (Functor)
@@ -84,6 +115,10 @@ runFetchConfig sources = do
then pure $ Right (result, origins, unusedWarnings <> warnings)
else pure $ Left errors
+-- | a list of all keys which will be read when running @runFetchConfig@ to
+-- produce a value of type @a@.
+--
+-- This runs inside the 'IO' monad, but does not do any actual IO.
configKeysOf :: forall a. Config a => IO [Key]
configKeysOf = do
let (Fetch m) = readConfig @a
@@ -92,8 +127,12 @@ configKeysOf = do
let keys = mapMaybe (\case {(NotPresent k) -> Just k; _ -> Nothing }) errors
pure keys
-
-
+-- | read an optional config value, resulting in a @Just@ if it is present
+-- and a @Nothing@ if it is not.
+--
+-- This is distinct from using 'readValue' to produce a value of type @Maybe a@:
+-- the latter will require the key to be present, but allow it to be @null@
+-- or similarly empty.
readOptionalValue :: forall a. ConfigValue a => Key -> Fetch (Maybe a)
readOptionalValue bareKey = Fetch $ \s1@FetcherState{..} -> do
@@ -114,7 +153,7 @@ readOptionalValue bareKey = Fetch $ \s1@FetcherState{..} -> do
pure (fst val, s1 { fetcherSources = sources
, fetcherOrigins = M.insertWith (<>) k (snd val) fetcherOrigins })
-
+-- | read in a config value, and produce an error if it is not present.
readRequiredValue :: ConfigValue a => Key -> Fetch a
readRequiredValue k =
let
@@ -128,6 +167,7 @@ readRequiredValue k =
pure (dummy, s { fetcherErrors = NotPresent (k `prefixedWith` fetcherPrefix s) : fetcherErrors s })
Just v -> pure (v, s)))
+-- | read in a config value, or give the given default value if it is not present.
readValue :: forall a. ConfigValue a => a -> Key -> Fetch a
readValue defaultValue k =
let
@@ -156,12 +196,19 @@ firstMatchInSources k (SomeSource (source, sourceState):sources) = do
Right _ ->
pure $ (eitherValue, SomeSource (source, newState)) : fmap (Left Shadowed ,) sources
+-- | read a nested set of configuration values, prefixed by a given key. This
+-- corresponds to nested objects in json.
readNested :: forall a. Config a => Key -> Fetch a
readNested (Key prefix') = Fetch $ \s1 -> do
let (Fetch nested) = readConfig @a
(config, s2) <- nested (s1 { fetcherPrefix = fetcherPrefix s1 <> NonEmpty.toList prefix' })
pure (config, s2 { fetcherPrefix = fetcherPrefix s1 })
+-- | same as 'readNested', but produce @Nothing@ if the nested keys are not present.
+-- This can be used for optionally configurable sub-systems or similar constructs.
+--
+-- If only some but not all keys of the nested configuration are given, this will
+-- produce an error.
readNestedOptional :: forall a. (Show a, Config a) => Key -> Fetch (Maybe a)
readNestedOptional (Key prefix) = Fetch $ \s1 -> do
let (Fetch nested) = readConfig @a
@@ -200,3 +247,112 @@ collectUnused sources = do
<&> fmap (\(Just a) -> Warning $ "Unused Keys " <> T.pack (show a))
. filter (\(Just a) -> not (null a))
. filter isJust
+
+
+{- $use
+
+This library models configuration files as a list of configuration 'Key's,
+for which values can be retrieved from generic sources, such as environment
+variables, a program's cli arguments, or a yaml (or json, etc.) file.
+
+As a simple example, assume a program interacting with some API. We want it
+to read the API's base url (falling back to a default value if it is not
+given) and an API key (and error out if it is missing) from its config:
+
+> data ProgramConfig =
+> { configBaseUrl :: URL
+> , configApiKey :: Text
+> }
+
+Then we can write an appropriate instance of 'Config' for it:
+
+> instance Config ProgramConfig where
+> readConfig = ProgramConfig
+> <$> readValue "http://example.org" [key|baseUrl|]
+> <*> readRequiredValue [key|apiKey|]
+
+'Config' is an instance of 'Applicative'. With the @ApplicativeDo@ language
+extension enabled, the above can be equivalently written as:
+
+> instance Config ProgramConfig where
+> readConfig = do
+> configBaseUrl <- readValue "http://example.org" [key|baseUrl|]
+> configApiKey <- readRequiredValue [key|apiKey|]
+> pure (ProgramConfig {..})
+
+Note that 'Config' is not a 'Monad', so we cannot inspect the config values here,
+or make the reading of further keys depend on the value of earlier ones. This is
+to enable introspection-like uses as in 'configKeysOf'.
+
+To read our config we must provide a non-empty list of sources. Functions to
+construct these live in the @Conftrack.Source.*@ modules; here we use
+'Conftrack.Source.Yaml.mkYamlFileSource' and 'Conftrack.Source.Env.mkEnvSource'
+(from "Conftrac.Source.Yaml" and "Conftrack.Source.Env" respectively) to read
+values from either a yaml file or environment variables:
+
+> main = do
+> result <- runFetchConfig
+> [ mkEnvSource "CONFTRACK"
+> , mkYamlFileSource [path|./config.yaml|]
+> ]
+> case result of
+> Left _ -> ..
+> Right (config, origins, warnings) -> ..
+
+Now we can read in a config file like
+
+> baseUrl: http://localhost/api/v1
+> apiKey: very-very-secret
+
+or from environment variables
+
+> CONFTRACK_BASEURL=http://localhost/api/v1
+> CONFTRACK_APIKEY=very-very-secret
+
+Of course, sources can be mixed: Perhaps we do not want to have our program's api
+key inside the configuration file. Then we can simply omit it there and provide it
+via the @CONFTRACK_APIKEY@ environment variable instead.
+
+== Multiple sources
+
+The order of sources given to 'runFetchConfig' matters: values given in earlier
+sources shadow values of the same key in all following sources.
+
+Thus even if we have
+
+> apiKey: will-not-be-used
+
+in our @config.yaml@ file, it will be ignored if the @CONFTRACK_APIKEY@ environment
+variable also has a value.
+
+== Keeping track of things
+
+Conftrack is written to always keep track of the configuration values it reads. In
+particular, it is intended to avoid frustrating questions of the kind "I have
+clearly set this config key in the file, why does my software not use it?".
+
+This is reflected in 'runFetchConfig'\'s return type: if it does not produce an error,
+it will not only return a set of config values, but also a map of 'Origin's and a list
+of 'Warning's indicating likely misconfiguration:
+
+> main = do
+> result <- runFetchConfig
+> [ mkEnvSource "CONFTRACK"
+> , mkYamlFileSource [path|./config.yaml|]
+> ]
+> case result of
+> Left _ -> ..
+> Right (config, origins, warnings) -> do
+> printConfigOrigins origins
+> ...
+
+May print something like this:
+
+> Environment variable CONFTRACK_APIKEY
+> apiKey = "very-very-secret"
+> YAML file ./config.yaml
+> baseUrl = "http://localhost/api/v1"
+
+It is recommended that programs making use of conftrack include a @--show-config@
+option (or a similar method of introspection) to help in debugging such cases.
+-}
diff --git a/src/Conftrack/Source.hs b/src/Conftrack/Source.hs
index ecfa20d..ab13172 100644
--- a/src/Conftrack/Source.hs
+++ b/src/Conftrack/Source.hs
@@ -12,11 +12,27 @@ import Control.Monad.State (StateT (..))
import Data.Text (Text)
+-- | An abstraction over "config sources". This might mean file formats,
+-- environment variables, or any other kind of format that can be seen as a
+-- key-value store.
class ConfigSource s where
+ -- | Some sources require state, e.g. to keep track of which values were
+ -- already read.
type SourceState s
+
+ -- | read a single value from the source.
fetchValue :: Key -> s -> StateT (SourceState s) IO (Either ConfigError (Value, Text))
+
+ -- | given @s@, determine if any keys are "left over" and were not used.
+ -- This is used to produce warnings for unknown configuration options;
+ -- since not all sources can support this, this function's return type
+ -- includes @Maybe@ and sources are free to return @Nothing@ if they
+ -- cannot determine if any unknown keys are present.
leftovers :: s -> StateT (SourceState s) IO (Maybe [Key])
+-- | An opaque type for any kind of config sources. Values of this type can be
+-- acquired from they @Conftrack.Source.*@ modules, or by implementing the
+-- 'ConfigSource' type class.
data SomeSource = forall source. ConfigSource source
=> SomeSource (source, SourceState source)
diff --git a/src/Conftrack/Source/Aeson.hs b/src/Conftrack/Source/Aeson.hs
index 97353d0..17ea4ee 100644
--- a/src/Conftrack/Source/Aeson.hs
+++ b/src/Conftrack/Source/Aeson.hs
@@ -7,7 +7,8 @@
{-# LANGUAGE RecordWildCards #-}
{-# LANGUAGE LambdaCase #-}
-module Conftrack.Source.Aeson (JsonSource(..), mkJsonSource, mkJsonSourceWith, mkJsonFileSource) where
+-- | Functions for producing sources reading from json strings or files, using the aeson library.
+module Conftrack.Source.Aeson (mkJsonSource, mkJsonSourceWith, mkJsonFileSource, JsonSource(..)) where
import Conftrack.Value (Key (..), ConfigError(..), Value (..), KeyPart)
import Conftrack.Source (SomeSource(..), ConfigSource (..))
@@ -37,13 +38,17 @@ data JsonSource = JsonSource
, jsonSourceDescription :: Text
} deriving (Show)
+-- | Make a source from an aeson value
mkJsonSource :: A.Value -> SomeSource
mkJsonSource value = mkJsonSourceWith ("JSON string " <> LT.toStrict (A.encodeToLazyText value)) value
+-- | same as 'mkJsonSource', but with an additional description to be shown
+-- in output of 'Conftrack.Pretty.printConfigOrigins'.
mkJsonSourceWith :: Text -> A.Value -> SomeSource
mkJsonSourceWith description value = SomeSource (source, [])
where source = JsonSource value description
+-- | Make a source from a json file.
mkJsonFileSource :: OsPath -> IO (Maybe SomeSource)
mkJsonFileSource path = do
bytes <- readFile path
diff --git a/src/Conftrack/Source/Trivial.hs b/src/Conftrack/Source/Trivial.hs
index d4151c2..842ca46 100644
--- a/src/Conftrack/Source/Trivial.hs
+++ b/src/Conftrack/Source/Trivial.hs
@@ -5,6 +5,7 @@
{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE OverloadedStrings #-}
+-- | A trivial source reading from a @Map Key Value@, only useful as a demonstration or for tests.
module Conftrack.Source.Trivial where
import Conftrack.Value (Key, Value(..), ConfigError(..))
diff --git a/src/Conftrack/Source/Yaml.hs b/src/Conftrack/Source/Yaml.hs
index 6adc798..4642922 100644
--- a/src/Conftrack/Source/Yaml.hs
+++ b/src/Conftrack/Source/Yaml.hs
@@ -6,6 +6,7 @@
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE UndecidableInstances #-}
+-- | Functions for producing sources reading from yaml strings or files, using the aeson library.
module Conftrack.Source.Yaml (YamlSource(..), mkYamlSource, mkYamlSourceWith, mkYamlFileSource) where
import Conftrack.Source (SomeSource(..), ConfigSource (..))
diff --git a/src/Conftrack/Value.hs b/src/Conftrack/Value.hs
index 3eda24a..1d6e6a7 100644
--- a/src/Conftrack/Value.hs
+++ b/src/Conftrack/Value.hs
@@ -8,7 +8,7 @@
{-# LANGUAGE LambdaCase #-}
{-# LANGUAGE DefaultSignatures #-}
-module Conftrack.Value (key, Value(..), ConfigError(..), Key(..), ConfigValue(..), Origin(..), KeyPart, prefixedWith) where
+module Conftrack.Value (key, Value(..), ConfigError(..), Key(..), ConfigValue(..), Origin(..), KeyPart, prefixedWith, withString) where
import Data.Text(Text)
import qualified Data.Text as T
@@ -21,9 +21,13 @@ import qualified Data.Text.Encoding as BS
import Language.Haskell.TH.Quote (QuasiQuoter(..))
import Language.Haskell.TH.Syntax (Lift(lift))
+-- | A generic value read from a config source, to be parsed into a more useful type
+-- (see the 'ConfigValue' class).
data Value =
ConfigString BS.ByteString
| ConfigInteger Integer
+ -- | A value which may be an integer, but the source cannot say for sure, e.g. because
+ -- its values are entirely untyped. Use 'withString' to handle such cases.
| ConfigMaybeInteger BS.ByteString Integer
| ConfigOther Text Text
| ConfigBool Bool
@@ -32,6 +36,14 @@ data Value =
type KeyPart = Text
+-- | A configuration key is a non-empty list of parts. By convention, these parts
+-- are separated by dots when written, although dots withing parts are not disallowed.
+--
+-- For writing values easily, consider enabling the @QuasiQuotes@ language extension
+-- to use 'key':
+--
+-- >>> [key|foo.bar|]
+-- foo.bar
newtype Key = Key (NonEmpty KeyPart)
deriving newtype (Eq, Ord)
deriving (Lift)
@@ -39,6 +51,7 @@ newtype Key = Key (NonEmpty KeyPart)
instance Show Key where
show (Key parts) = T.unpack (T.intercalate "." (NonEmpty.toList parts))
+-- | to write values of 'Key' easily
key :: QuasiQuoter
key = QuasiQuoter
{ quoteExp = lift . Key . NonEmpty.fromList . T.splitOn "." . T.pack
@@ -57,8 +70,11 @@ data ConfigError =
| Shadowed
deriving Show
+-- | Values which can be read from a config source must implement this class
class ConfigValue a where
fromConfig :: Value -> Either ConfigError a
+ -- | optionally, a function to pretty-print values of this type, used by the
+ -- functions of "Conftrack.Pretty". If not given, defaults to @a@'s 'Show' instance.
prettyValue :: a -> Text
default prettyValue :: Show a => a -> Text